It is known to disperse micro-particles (i.e. particles generally defined as being of between 0.1 and 100 microns (μm) in size) of a hard material such as nitrides, carbides (e.g. silicon carbide (SiC) and tungsten carbide (WC)), borides and oxides in a polymer matrix to form polymeric bearing coatings for use in automotive environments. This can increase the load-carrying capacity and decrease the wear rate of bearing elements. These hard micro-particles typically have an average particle size in the range 1-10 microns. For example, micro-particles such as nitrides, carbides (e.g. silicon carbide (SiC) and tungsten carbide (WC)), borides and oxides are commonly used to improve the wear resistance of bearing elements coated with a polymer-based sliding layer (or ‘running layer’ or ‘overlayer’).
However, the inventors' tests have shown that there may be limitations associated with the use of hard micro-particles in sliding layer materials, particularly for bearings coated with a polymer-based sliding layer for automotive applications. The tests have indicated that inclusion of hard micro-particles in sliding layer materials may promote fatigue, particularly of polymeric bearing coatings, which may be undesirable as it may increase wear rate and decreases bearing lifetime.
The inventors have also identified that inclusion of hard micro-particles in sliding layer materials necessitates a certain minimum thickness of the sliding layer which may be undesirable as it may increase friction by comparison with thinner polymeric bearing coatings. The inventors have further identified that inclusion of hard micro-particles in sliding layer materials may lead to localised ‘hard spots’ in the sliding layer, particularly if the layer thickness is insufficient, which may lead to uneven wear and so may be undesirable.
The invention aims to address one or more of these problems.